The Almighty Battery

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< Deep-cycle batteries from Trojan battery Company

Another difference is that voltage isn't a life-or-death factor with ni-cads. In fact, it is recommended that they be completely drained a couple of times a year. (If you buy new cells of either type, they must be cycled as many as 50 times to get up to running capacity).

A problem you may face with ni-cads is that much of the alternative energy equipment available is built only with the more popular lead-acids in mind. Specifically, ni-cads must be charged to a voltage that's higher than many inverters, chargers, and the like can handle.

Finally, the two types of batteries also act differently. The lead-acids charge with the voltage rising steadily and discharge similarly, with a steady voltage drop. Ni-cads charge with the voltage rising somewhat steadily, but once charged, they maintain a relatively high voltage during use until they begin to run low, at which time the voltage plunges.

Aside from choosing the type of battery most useful for your situation, you must also properly size the bank for your needs. An undersized bank will have you running constantly to the volt meter and will leave you frequently frustrated.

< A bank's enclosure should be ventilated and isolated from the home.

The first step to putting together a relatively effortless system is to get a realistic idea of your usage and storage needs. If you've been living off-grid with a bank of batteries that doesn't cut it, you'll already have a good idea of what you've got and how long you can run on it. If you are just preparing to take the plunge, you should start by estimating your general usage. Get this estimate by listing each appliance you rely on and how much you use it every day, on average. Then assign a number to each item reflecting either the amps or the watts it draws, which should be listed somewhere on the appliance.

The storage capacity of your bank will be rated in amp/hours, usually though not always-at a 20 amp/hour rating. (While I'll explain the basics of this here, be warned that it can be confusing. Ask a technician at any of the major suppliers to review with you anything that remains unclear.) Bank capacity refers to how many hours of storage are available at a specified, constant draw. For instance, if you have four floor-scrubber lead-acid batteries interconnected with an amp/hour rating of 370 each at the 20 amp/hour rating, then your bank is 1,480 amp/hours. To understand how much storage this is, take the rating (20 amp/hours) and divide it into the available storage (1,480), which gives you how many hours are available (74). In other words, if 20 amps are drawn continually from the bank, each hour amounts to 20 amp/hours drawn, and the bank above can handle this load for 74 hours.

The tricky part is figuring your general usage list in terms of amp/hour capacity. If you are running an inverter and the appliances on your list are rated at 120 volts, then the wattage and amp figures you've jotted down must be changed to the voltage of your battery system to jibe with your amp/hour rating. The formula is watts=volts x amps. If a hair dryer is rated at 1,200 watts at 120 volts, it will draw 10 amps (1,200 watts=120 volts x 10 amps). If you have a battery bank rated at 12 volts, then you must change this figure over to accommodate the voltage of your system: the wattage will remain the same, the voltage is changed, which in turn changes the amperage (1,200 watts=12 volts x 100 amps). So you know the load to run the hair dryer will be 100 amps, but anything run through an in inverter needs more energy than the device alone requires, because inverters have certain inefficiencies that are different for each unit. You'll need to either research the figure for your equipment, or else just understand that the total of your daily usage sheet will be a low figure if many of your appliances require inverted power. Also, the batteries themselves suffer from inefficiency. Adding 30% to your overall needs is the generally accepted rule when summing the total usage figure.

With photo-voltaic's and wind power, it's best to have a battery bank that stores enough current to supply your regular needs for a week. In most places, this will allow you to live at your regular pace through spells when conditions keep you from making power. Also, sizing a bank this way will assure that you have one large enough to maintain a high, stable voltage when starting and running large appliances. If you're lucky enough to have hydroelectric power, your storage needs may be smaller than those with solar or wind systems, since you get steady, reliable current production.

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